The State-of-the-practice of Geotechnical Engineering in Taiwan and Hong Kong
Foundation Design & Construction in Hong Kong – Present & Beyond?
Daman Lee – Ove Arup & Partners Hong Kong Limited W.K. Pun – Geotechnical Engineering Office, CEDD Arthur So – China State Construction Engineering (Hong Kong) Ltd. C.C. Wai – Gammon Construction Limited
Our Theme • A brief look at where we are • Issues still bugging us • Where do we go now?
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Common Foundation Types in Hong Kong Steel H piles Large diameter bored pile
Mini-piles
1MN
4MN
6MN
Driven
Up to 100MN
Prebored
New Publications in Hong Kong 1 / 2006
2
Progress in Last 5 Years ……
Li et al (2000) “Design of Deep Foundations in Hong Kong – Time for change?”
(1) End Bearing Bored Piles on Rock • Predominately using presumptive values • Bearing pressures generally regarded as very conservative
GEO Publication 1/2006 provides an alternative approach
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(2) Bell-out / Rock Socket • Many queried the actual effectiveness of bell-out (eg Lumb)
Typically 10MPa
• A convenient way to address bearing pressure being too conservative?
Half-addressed by Code of Practice for Foundation (BD, 2005) Typically 5 – 7.5 MPa
(3) H-pile Driven to Rock • Shallow rockhead • Pile driven into rock • Loading tests tend to produce positive results Shallow rockhead
Addressed by Code of Practice for Foundation (BD, 2005)
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(4) Negative Skin Friction • Drag on piles due to settling ground • Transient loads results in temporary settlement • Need to consider a combined Addressed by Code load case of both?
of Practice for Foundation (BD, 2005) Settling ground
(5) Issues Requiring More Progress
• Use of hydraulic hammer to achieve final set • Bored pile on rock – how to deal with soft materials at the interface
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Bored Piles
Design & Construction Issues with Bored Piles • Bearing pressure on rocks • Combined end bearing and rock socket • 45º load spread • Pile base imperfections
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Bearing Pressures on Rock • First appear in 1990 • Considered to be very conservative • Many full-scale pile loading tests had been undertaken since • Used until 2005
30
Loading Tests by West Rail
(14)
(13)
• In the Technical Memorandum, 50% increase in bearing stresses were approved by BD • Various other loading tests also support this
End bearing stress qb (MPa)
(2)
• Presented in many previous occasions
(7.5)
25 (2-3)
West Rail
(15)
20 (30)
(11) (7)
(130
PNAP141
15 (11)
(11) (12)
1.0xUCS 10 (1)
(30)
Breccia
5 (1)
0 0
50
100
150
200
250
Average unconfined compressive strength (MPa)
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Bearing Stresses in the New Code of Practice for Foundations (BD, 2005)
• A new prescriptive bearing stress for highly to completely decomposed rocks • No change to the other categories • A new prescriptive bearing stress for fresh rock – note the requirement of 100% total core recovery (TCR) and no weathered joints
End Bearing on Rock – Alternative to Presumptive Values Mobilised Bearing Pressure q (MPa) 30
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Bearing pressure thatBearing can induce pressure that settlement ~1% can induceofsettlement of of the about pile 1% dia.of the pile diameter at the at the pilebase base.
Mobilized Bearing Pressure, qa (MPa)
All ow abl e 20 Be ari ng Pre 15 ssu re (M Pa)
14.5 12.5 10
10 7.5
Recommended Recommended allowable allowable bearing pressure bearing pressure 88
5
5
3
3
0 0
10
20
30
40
50
60
70
80
90
100
Rock Mass Rating (RMR)
Rock Mass Rating (RMR) Legend: ? = Bearing pressure substantially mobilised ? = Degree of mobilisation of bearing pressure unknown
Extracted from GEO Publication 1/2006 (In Press)
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• Data published in recent years
Mobilized Shaft Resistance in Rock, τ (kPa)
The Use of Rock Socket
10000
Extracted from GEO Publication 1/2006 (In Press)
• Many used Osterberg Cell at pile base direct measurement of socket behaviour Ultimate socket friction, τs (kPa) Ultimate shaft friction τs (kPa)
10000
1000
τs = 0.2 σc 0.5
100 1
10
1000
100
Uniaxial Compressive Strength Rock, q c (MPa) Uniaxial Compressive Strengthof of Rock, σc (MPa)
Legend: ? = Shaft resistance substantially mobilised ? = Degree of mobilisation of shaft resistance unknown
τs=
√qu 0.3
Figure 19 Mobilized Shaft Resistance in Rock Sockets
Williams & Pells
1000
Horvath et al (1980)
Geoguide 1/96 GEO Publicati on No. Code of1/96 Practice 2004
Horvath et al (1980) Long & Collins (1998) Radhakrishnan & Leung (1989) Williams & Pells (1980) Glos & Briggs (1983) Shiu & Chung (1994) Lam et al (1991) Arup tests in HK KCRC West Rail tests Zhan & Yin (2004) Incheon 2nd Bridge (2005)
Extracted from West Rail papers
100 1
10
100
1000
Uniaxial Compressive Strength, Unconfined compressive strengthqof rock (MPa) u (MPa)
Load Deflection Behaviour of Rock Sockets • This is where • The bullet points • Are to go
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Combined Rock Socket and End Bearing • Rock socket behaves in a ductile manner • Should allow direct combination of both without the need of further loading test • Provide a robust alternative to the use of bell-out if the socket length to pile diameter ratio is around 3 • Max ratio allowed by BD (2005) is 2 (or 6m whichever is less)
45º Load Spread
Under the strange rule, there is no need for any load spread check in this case!
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Pile Toe Imperfections • Major issue a few years ago • Less so nowadays, but not completely resolved • The use of pressure grouting is still routinely done as a remedial measure
Study by the HK Contractor Association (2001-2002) • Thin layer of soft materials at pile base does not always require remedial works • Factors to be considered: • A single pile or pile group? • Probing at centre of pile or edge of pile? • “unbound” aggregate, soil inclusions or coreloss
• Suggested a rational approach to the problem
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Prescriptive Approach – An Example Interface Investigation • Soft This is where Layer Thickness S ≤ 100 N/A
Remedial Works/ Proposal Flush clean + • The bullet pointsnormal grout 100 < S ≤ 150 N/A Flush clean + • Are to go pressure grout 150 < S ≤ 200 Sonic test (Fan Flush clean + shape) with pressure grout satisfactory results Unsatisfactory results
Further Investigation N/A
N/A N/A
N/A
coring for second hole S ≤ 100
N/A
100 < S ≤ 150 150 < S ≤ 200
S > 200
Remedial Works/Proposal N/A
Flush clean + normal grout Flush clean + pressure grout Pressure jet clean + pressure grout
Concrete pile Coring tube
Thickness Of soft Material - s
Further investigation + submit remedial proposal
Driven H-piles
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Design & Construction Issues with Driven H-piles • Pile driving formula • Final set problems • Whipping of piles • Long piles
Shall We Keep Using Hiley Formulae? • Different views • “Simple is beautiful” vs “Too simple, sometimes …..” • If we were to vote …..
examples
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The Driving Formulae • Hiley (1925) • In 1960, more than 450 formulae of slight variations to Hiley • HKCA (1994) – lumped various efficiency terms into a single factor Kh • HKCA (2004) – uses energy measured from PDA • Gradually increasing experience in modelling the efficiency of hydraulic hammers in Hiley Formula • Difficulties encountered in long piles
Development of the Wave Equation • Isaacs (1931) – First used 1-D stress wave theory in pile driving analysis • Smith (1960) – forms the basis of modern wave equation analysis • Development of bonded resistance strain gauges • Research work at Case Institute of Technology • CAPWAP, GRLWEAP • At present, limited to detect pile defects, measure hammer efficiency
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A State of Confusion • Different departments have different approaches • ASD approach: Use of CAPWAP to determine pile capacity and calibrate against parameters in the Hiley Formula • Private projects: essentially HKCA (1994), with trial piles to establish kh and PDA/CAPWAP • Contractors do not know how small the set needs to be in order to pass the loading tests
Long Piles – Big Hammers • Ideal Situation: SPT N > 200 at around 30-40m; set usually achieved 3-5m into the saprolite • For longer piles, the length effect of Hiley Formulae starts to show • Even 20t hammers dropping from 4m is not enough • So-called “Driving to refusal”
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Required Set for Long Piles at a Particular Site (over 4000 piles, 35-80m long)
Hypothetical Allowable Set
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Whipping of Piles
Why Does This Happen? • Happens mostly in sites with soft deposits in the upper layers (eg reclamation) • Let go in its weaker axis with insufficient lateral restraint during driving • Some contractors attempt to avoid this by reducing the drop height and carry out final set a few days later
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Signs of Whipping from Shaft Shortening Measurements
• Static load tests carried out at a particular site with thick layer of soft soils • Signs of whipping
More to Tackle ………………. • Residual settlement on pile loading tests • Use of pile raft (settlement reducing piles) • Use of base-grouting in competent soils • Ultimate limit state design? • etc etc • Encourage rational designs when time and resources are available
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Thank You
Typical Set Table TYPE OF PILE UNIT WEIGHT OF PILE DRIVING RESISTANCE TYPE OF HAMMER WEIGHT OF HAMMER OR RAM WEIGHT OF PILE HEAD HELMET ENERGY OUTPUT PER BLOW (MAX) (W*h*k*ٛ )/R-C/2
S=
: : (R) : : (W) : (WH) : (W*h) :
305x305x223 kg/m 223 kg/m 7096 kN <=3548x2 Drop Hammer (DH-06) 20.45 ton 0.59 ton 642 kN.m
WHERE
C = Cp + Cq + Cc
DROP OF HAMMER EFFICIENCY OF HAMMER COEFFICIENT OF RESTITUTION TEMPORARY COMPRESSION OF PILE HEAD
200.6 kN 5.79 kN
Efficiency of blow : = W+P*e^2 W+P
WHERE
(h) : (k) : (e) : (Cc):
3.2 m 0.6 0.5 (Steel Anvil) 1.5 mm
P = WEIGHT OF PILE AND HELMET
Max. Allowable Penetration (mm) For Last 10 Blows Table Pile Length (m) 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
Blow Efficiency
Temporary Compression Cp+Cq (mm) 61 49 48
62 50 49 47 46 44 43
63 49 48 47 45 44 42 41 39 38
64 49 47 46 44 43 42 40 39 37 36 34 33
65 50 49 47 46 44 42 41 39 38 37 35 34 32 31 29 28
66 50 49 47 45 44 42 41 39 37 36 34 33 32 30 29 27 26 -
67 49 47 45 44 42 40 39 37 36 34 32 31 29 28 27 25 -
68 49 47 46 44 42 40 39 37 35 34 32 31 29 27 26 -
69 46 44 42 41 39 37 35 34 32 30 29 27 26 -
70 41 39 37 36 34 32 30 29 27 25 -
71 36 34 32 31 29 27 25 -
72 31 29 27 26 -
73 26 -
74 -
ٛ 0.733 0.730 0.726 0.723 0.720 0.717 0.714 0.711 0.707 0.704 0.701 0.699 0.696 0.693 0.690 0.687 0.684 0.682 0.679 0.676 0.674 0.671 0.668 0.666 0.663
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Examples B.H. Fellenius
E. Blackett
return
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